Multi-layer micro structurefor sensing substance

ABSTRACT

A micro structure for sensing a substance using light scattering includes a substrate, a first layer on the substrate, wherein the first layer comprises a metallic material, a second layer over the first layer, and a mask layer over the second layer. A plurality of nano holes are formed through the mask layer and the second layer, wherein the plurality of holes are defined in part by internal surfaces on the second layer and the mask layer. Two or more structure layers are formed on the mask layer and the internal surfaces in the plurality of holes. The two or more structure layers comprise different material compositions.

The present patent application is a Continuation-in-Part patent application of and claims priority to commonly assigned pending U.S. patent application Ser. No. 11/754,912, entitled “Light scattering device having multi-layer micro structure” filed May 29, 2007 by the same inventors, the content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present application relates to Raman scattering devices including a sensing substrate having a micro structure.

A limitation for conventional Raman spectroscopy is the weak Raman scattering signals for trace chemical detection. Techniques for increasing Raman scattering signals include Surface-Enhanced Raman Spectroscopy (SERS) and Surface-Enhanced Resonance Raman Spectroscopy (SERRS). Molecules of a trace chemical can be adsorbed on micro structural surfaces. It was discovered that noble metals on the surfaces of the micro structures could enhance the Raman scattering signal.

There remains a need for a micro structure that can be fabricated by well-controlled manufacturing techniques. There is also a need for a Raman scattering device having non-contaminated micro structures allow Raman scattering measurement to be conducted in the field.

SUMMARY OF THE INVENTION

In a general aspect, the present application relates to a micro structure for sensing a substance using light scattering. The micro structure includes a substrate; a first layer on the substrate, wherein the first layer comprises a metallic material; a second layer over the first layer; a mask layer over the second layer, wherein a plurality of holes are formed through the mask layer and the second layer, wherein the plurality of holes are defined in part by internal surfaces on the second layer and the mask layer, wherein widths of the plurality of holes are in the range of about 1 nm and about 1,000 nm; and two or more structure layers formed on the mask layer and the internal surfaces in the plurality of holes, wherein the two or more structure layers comprise different material compositions.

In another general aspect, the present application relates to a micro structure for sensing a substance using light scattering. The micro structure includes a substrate; a first layer on the substrate; a second layer over the first layer; a mask layer over the second layer, wherein a plurality of holes are formed through the mask layer and the second layer, wherein the plurality of holes are defined in part by internal surfaces on the second metal and the mask layer, wherein widths of the plurality of holes are in the range of about 1 nm and about 1,000 nm; and one or more structure layers formed on the mask layer and the internal surfaces in the plurality of holes, wherein the structure layer comprises at least one of a polymeric material, a metallic material, or an oxide material.

In another general aspect, the present application relates to a method for making a micro structure for sensing a substance using light scattering. The method includes forming a first layer on a substrate; forming a second layer over the first layer; forming a mask layer over the first layer; forming a plurality of holes are formed in the mask layer and the first layer, wherein the plurality of holes are defined in part by internal surfaces on the second layer and the mask layer, wherein widths of the plurality of holes are in the range of about 1 nm and about 1,000 nm; and forming one or more structure layers formed on the mask layer and the internal surfaces in the plurality of holes.

In a general aspect, the present application relates to a micro structure including a silicon substrate; an adhesion layer on the silicon substrate; a bias layer on the adhesion layer; and two or more structure layers on the adhesion layer, wherein the two or more structure layers comprise different material compositions and a plurality of holes through at least two of the two or more of structure layers, wherein widths of the plurality of holes are in the range of 0.5-500 nm.

In another general aspect, the present application relates to a micro structure including a silicon substrate; an adhesion layer on the silicon substrate; a bias layer on the adhesion layer; and a plurality of columns on the bias layer, wherein at least one of the plurality of columns or holes comprises two or more structure layers having different material compositions and have widths in the range of in the range of 0.5-500 nm.

In another general aspect, the present application relates to a method for fabricating a micro structure. The method includes forming an adhesion layer on a substrate; forming a thermal bias layer on the adhesion layer; two or more structure layers on the adhesion layer, wherein the two or more structure layers comprise different material compositions; forming an upper layer on the two or more structure layers; producing recesses or protrusions on the upper layer; removing portions of the upper layer to produce a mask having a plurality of openings; and forming a plurality of holes in the two or more structure layers or a plurality of columns having the two or more structure layers by removing portions of the two or more structure layers through the openings in the mask, wherein widths of the plurality of holes or columns are in the range of in the range of 0.5-500 nm, such as 5-200 nm.

In another general aspect, the present application relates to a method for a micro structure. The method includes forming an adhesion layer on a substrate; forming a bias layer on the adhesion layer; two or more structure layers having different material compositions on the adhesion layer; forming an upper layer on the two or more structure layers, wherein the upper layer comprises a metallic material; anodizing at least a portion of the upper layer to produce a mask having a plurality of openings; and forming a plurality of holes in the two or more structure layers or a plurality of columns having the two or more structure layers by removing portions of the two or more structure layers through the openings in the mask, wherein the widths of the plurality of holes or columns are in the range of in the range of 0.5-500 nm.

Implementations of the system may include one or more of the following. The one or more structure layers can include a material selected from the group consisting of Ti, Ni, Co, Ag, Au, Pd, Cu, Pt, Sn, Al, Fe, Cr, Rh, Ru, SiO₂, Al₂O₃, ZnO, TiO₂, SiO₂, Si₃N₄, Ta₂O₅, Zn oxide, Fe oxide, Sn oxide, Sb oxide, Ag oxide, Au oxide, and polymethyl methacrylate. The one or more structure layers can include a material selected from the group consisting of GaAs, ZnS, CdS, InGaN, InGaN/GaN, AlGaAs, InAgAs, GaAs/GaAlAs, GaN, 4H SiC, AlN, GaN, AlGaN/GaN, InP, InAlAs/InGaAs, Cs, Rb, InAs, AlSb/InAs, AlGaAs/InGaAs, InAlAs, InGaP, SiGe, C, Si, and SiC. The first layer can include material selected from a group consisting of Ti, Ni, or Co. The second layer can include Ti, Ni, Co, Cr, Al, or Zn. The micro structure can further include a third layer between the second layer and the mask layer, wherein the plurality of holes are formed through the mask layer, the third layer, and the second layer, wherein the third layer comprises Ti, Ni, Co, Ag, Au, Pd, Cu, Pt, Sn, Al, Fe, Cr, Rh, Ru, SiO₂, Al₂O₃, ZnO, TiO₂, SiO₂, Si₃N₄, Ta₂O₅, Zn oxide, Fe oxide, Sn oxide, Sb oxide, Ag oxide, Au oxide, or polymethyl methacrylate. The plurality of holes can be defined in part by an upper surface of the first layer. The micro structure can further include bias layer between the first layer and the second layer, wherein the plurality of holes are formed through the mask layer, the second layer, and the bias layer. The bias layer can receive a bias voltage to enhance adsorption of molecules on the one or more surfaces in the plurality of holes for Raman scattering sensing of trace chemicals. One or more surfaces of the one or more structure layers can adsorb molecules of a trace chemical for detection of the trace chemical. The molecules can be adsorbed from a liquid, sol gel, a gas, an aerosol, or a mixture of liquid, sol gel, gas, and aerosol. At least some of the plurality of holes can be distributed substantially in a periodic array in the one or more structure layers. The center-to-center spacing between the adjacent holes in the plurality of holes can be in the range of about 1 nm and about 1000 nm. Depths of the plurality of holes can be in the range of 1 nm and 2000 nm. The mask layer can include aluminum, Al oxide, or PMMA.

Embodiments may include one or more of the following advantages. The disclosed systems and methods may enhance the intensity of the scattered light for detecting trace chemicals. Different material compositions in the multiple layers in a multi-layer nano structure also allow different types of chemical molecules to be adsorbed to the surfaces of the nano structures, which can enable the detection of more than one type of trace chemicals.

These and other objects and advantages of the present application will become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment, which is illustrated in the various drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a three layer structure to start the process for fabricating a nano-structured surface.

FIGS. 2A to 2C are respectively a cross sectional view of an assisting layer with holes formed by anodization process, a top view and a side cross sectional view along a horizontal line over the top view of FIG. 2B.

FIG. 3 shows a cross sectional view of the nano-structured surface after performing a chemical etch or CMP process.

FIG. 4 shows a cross sectional view of the nano-structure surface after removing the barrier layer at the bottom of the holes and etching down to the conducting layer by chemical etching.

FIGS. 5A and 5B show respectively a noble metal deposited on top of the nano-structured surface then followed by removing the noble metal film from the top layer.

FIG. 6 shows the side cross sectional view of the nano-structured surface with noble metal rods after the oxide layer is removed.

FIGS. 7A to 7H are a series of cross sectional views and top views to show an alternate processing method to form nano-structure surface in accordance with the present application.

FIG. 8 is a perspective view for showing a SERS or SERRS probe contained in air tight sealed probe cell.

FIG. 9 is a perspective view for showing a roller for moving and exposing the nano-structured sensing surface to incident laser beam as implemented in a SERS or SERRS probe of FIG. 7.

FIG. 10 is a perspective view for showing the surface packaging configuration of the nano-structured surface.

FIG. 11 is an alternate embodiment with a nano-structured surface with pocket sealed and disposed as thin film on a rotary wheel.

FIGS. 12A-12C are cross-sectional views of multi-layer structures from which nano structures can be fabricated.

FIG. 13A is a cross-sectional view illustrating the relative positions of a mold and the multi-layer structure of FIG. 12A before imprinting.

FIG. 13B is a bottom view of the mold of FIG. 13A.

FIG. 13C is a cross-sectional view illustrating the relative positions of the mold and the multi-layer structure of FIG. 12A during imprinting.

FIG. 13D is a cross-sectional view illustrating the impressions formed on the upper surface of the multi-layer structure of FIG. 12A after imprinting.

FIG. 13E is a cross-sectional view illustrating the formation of a mask on the multi-layer structure of FIG. 12A.

FIG. 13F is a top view of the nano holes formed in the multi-layer structure.

FIG. 13G is a cross-sectional view along the line A-A in FIG. 13F.

FIG. 13H is a cross-sectional view illustrating the multi-layer structure having the nano holes after the mask is removed.

FIG. 14A is a bottom view of a mold for nano-column formation.

FIG. 14B is a top view of the nano columns of FIG. 14A.

FIG. 14C is a cross-sectional view of the nano columns along the line A-A in FIG. 14B.

FIG. 15A is a cross-sectional view of a multi-layer structure.

FIG. 15B is a cross-sectional view of the multi-layer structure of FIG. 15A after anodization of the upper layer.

FIG. 15C is a top view of the nano holes formed by etching through a mask formed by the anodization.

FIG. 15D is a cross-sectional view of nano holes along the line A-A in FIG. 15C.

FIG. 16 is a cross-sectional view of another multi-layer structure suitable for constructing a nano structure.

FIG. 17A is a cross-sectional view illustrating the relative positions of a mold and the multi-layer structure shown in FIG. 16 before imprinting.

FIG. 17B is a bottom view of the mold of FIG. 17A.

FIG. 17C is a cross-sectional view of the mold and the multi-layer structure during imprinting.

FIG. 17D is a cross-sectional view illustrating the impressions formed on the upper surface of the multi-layer structure after imprinting.

FIG. 17E is a cross-sectional view illustrating the formation of a mask on the multi-layer structure of FIG. 16.

FIG. 17F is a top view of the nano holes formed in the multi-layer structure.

FIG. 17G is a cross-sectional view along the line A-A in FIG. 17F.

FIG. 18 is a cross-sectional view of a multilayer nano structure.

DETAILED DESCRIPTION

Referring to FIGS. 1 to 6 for a series of processing steps to fabricate a nano-structured noble metal surface of this application, FIG. 1 shows a two-layer structure with n-type (100) silicon wafer (3-8 Ω-cm) or oxidized (30-50 nm SiO₂) p-type (100) silicon wafers (5-10 mΩ-cm), an electrically and thermally conductive layer 110 deposited on a (100) silicon substrate 105. The thickness of the conductive layer 110, such as Ti and Ni, is optimized to provide i) best adhesion to a subsequently deposited noble metal film, such as Ag, or Au film, etc., ii) electrical conductive film to apply electrical bias to sensing surface in field application, iii) thermal conductive layer to apply lower temperature of sensing surface, iv) thermal heat sink film to conduct localized heat generated by excitation of a light source. The thickness of this metal film is usually controlled in the range of 50 Å-50,000 Å, or 100 Å-1,000 Å. Then an aluminum layer 115 with purity of 99.999% and thickness in the range of 2,000 Å-10.0 μm is deposited on top of the conductive layer 110. Anodization is performed to produce a porous structure in a form of porous aluminum oxide layer 115 as that shown in FIG. 2A. FIG. 2B is a top view of the porous structure formed on the aluminum oxide layer 115 wherein the porous structure includes a plurality of pores 112 surrounded by pore wall 114 with the cross section view along a horizontal line A-A shown in FIG. 2C. Then wet oxide etch process is carried out in FIG. 3 to remove both top porous Al₂O₃ layer and barrier layer. A second anodization process is carried out to consume all Al metal so that the barrier layer and top porous Al₂O₃ layer are right above the conductive metal layer. In FIG. 4, an oxide etching is carried out to remove the barrier layer at the bottom of the pores and to widen the pore diameter. After the completion of the wet etch process, the pores 112 are extended downward to reach the conductive layer. The thickness of the resulted porous oxide layer can be controlled by controlling the processing parameters of aluminum physical vapor deposition (PVD); anodization and the subsequent wet etch processes. The self-assembled pore structure is naturally formed with a hexagonal array. The pore diameter (d) depends on applied anodization voltage, current density and electrolyte, and the subsequent pore widening wet etch process; while the inter-pore distance (D) depends on applied anodization V, i and electrolyte. Alternatively, a second anodization process can be carried out to consume part of Al metal film, so that the barrier layer and top porous Al₂O₃ layer are above the Al metal layer.

A noble metal, e.g., Ag layer 120 is deposited on top of the porous layer 115 in FIG. 5A and filling the pores 112 by bias PVD process or plating method. In FIG. 5B, the top layer of the noble metal 120 is removed with the noble metal 120-N filled the pores 112. Another wet metal etch or CMP process is carried out to further control height of the noble metal 120-N filling the pores. In FIG. 6, the aluminum oxide 115 and the residue aluminum film 115-AL at the bottom of the porous aluminum layer 115 are removed, then the noble metal rod array with the rod diameter d 120-N left with controlled height (H) and a well-defined nano-structured center to center distance (D) thus completing the fabrication of a noble metal nano-structured surface 100.

The geometries of the masks applied in the above-described fabrication processes are designed to match the expected size of the sensing chip and the area of the metal pad, which locates at the corner of edge of a chip. For field applications, the chemical detection sensing chips are formed as packaged sensing chips by applying different semiconductor packaging technologies, e.g., wire-bonding, flip-chips, tape and rail, system-on chip (SOC), etc.

As disclosed in FIGS. 1-6, this application provides a novel method of using aluminum material and anodization method to creating nano-scaled porous structure on a silicon substrate with a conductive layer of metal coating or other compatible material surfaces. The layer thickness and the novel geometrical parameters of the nano-structure are precisely controllable because the processing parameters of the aluminum PVD, the anodization, and wet etch and the CMP processes are well known in the art. A precisely controllable chemical etching process is applied to remove the barrier layer at the bottom surface of the pores. The porous aluminum oxide layer is applied as a hard mask for depositing the noble metal into the pores 114 and then the residue aluminum film and the porous aluminum oxide are removed to expose the noble metal rods with well-controlled height H and distance D between the rods by controlling the anodizing processes on the aluminum layer, the chemical process, or the CMP processes. The present application thus provides a nano-structured surface fabricated by these novel processing steps on a (100) silicon substrate. By using the nano-structured surface, a Raman scattering band around 520 cm⁻¹ from silicon substrate can be applied as the internal reference to assist alignment during Raman experiment and calibrate spectrum frequency and intensity in the field application. A voltage may be applied to the nano-structure sensing surface through the conductive layer 110 for the purpose of attracting electrically charged trace chemical in the form of electrically charged molecular clusters, e.g., either negatively or positively charged particles depending on the sensing applications. Furthermore, the conductive layer 110 can also be cooled to a lower temperature below a normal room temperature to further enhance surface adsorption of molecules of interest.

Referring to FIGS. 7A to 7F for a series of processing steps to fabricate another nano-structured noble metal surface of this application, FIG. 7A shows a two-layer structure. The two-layer structure has an electrically and thermally conductive layer 135 deposited on top of a silicon substrate 130. In some embodiments, the conductive layer 135 may be titanium (Ti) or nickel (Ni) layer. The substrate 130 may be an n-type silicon substrate (3-8 Ωcm) or oxidized (30-50 nm SiO₂) p-type silicon wafers (5-10 mΩcm). The thickness of the conductive metal film 135 can be controlled in the range of 100 Å-1,000 Å and optimized to provide best adhesion to a noble metal layer to be deposited as described next. The thickness of the metal layer 135 is also optimized for applying an electrical bias to the sensing surface for trace chemical detection and further for applying a lower temperature to the sensing surface to enhance sensitivity of trace chemical detection. In FIG. 7B, a noble metal layer 140 is deposited on the conductive layer 135. The noble metal layer can be made of silver, with a thickness of 0.5-5,000 nm, or 10-200 nm. In FIG. 7C, a second layer 145, e.g., an aluminum layer 145 with a thickness in the range of 0.5-10.0 μm is deposited on the noble metal layer 140. Anodization is next carried out, which transforms the aluminum layer 145 an aluminum oxide layer 150 with pores 148. The self-assembled hexagon-shaped nano pores 148 are surrounded by hexagon-shaped pore wall 149. After removing top anodized layer and the barrier layer by a wet chemical process, a second anodization process is carried out to convert all Al metal to aluminum oxide. Then a wet etch process is performed to widen the pores 148 at the bottom of the pores 148. As that shown in FIG. 7F, as the wet etch process proceeds, the pores 148 are widened and the walls 149 surrounding the pore become thinner. The etch process can be controlled to either form a plurality of nano-holes 148 surrounded by wall 149 or the pores 148 can be widen such that the pores 148 tangentially touch each other. A plurality of quasi-triangle nano rods 150 are formed after the etch process is completed.

In FIG. 7G, the noble metal layer 140 is etched down and the pores 148 are extended downward to reach the conductive titanium layer 135. In FIG. 7H, a wet oxide etch is performed to remove the aluminum oxide followed by a wet metal etch to remove the aluminum residue at the bottom of the pores 148. The aluminum oxide 115 and the residue aluminum film 115-AL at the bottom of the porous aluminum layer 115 are removed to leave an array of nano rod 150 having a space 147 in between the nano rods 150.

The above described method results in nano rods 150 with quasi-triangle shape. The coordination number is three. The advantages of this method over the embodiment shown in FIGS. 1 to 6 as that discussed above are i) better film adhesion between the Ti layer 135 and the Ag layer 140, ii) shorter inter-nano-rod distance, i.e., D_(A) as shown in FIG. 7F, with about 40% reduction comparing to D_(B) if all other process conditions are the same, iii) the height of the nano rods 150 can be well-controlled by Ag PVD in film thickness through the whole wafer containing at least several hundred or even several thousand devices.

According to above descriptions, the self-assembled nano sensing surface is formed that an array of nano rods 150 or a hexagonal array of nano holes 148 wherein each Ag nano-rod or nano-hole array are spatially isolated from each other.

The nano-rod array dimension size can be well controlled by processes mentioned above. Specifically, the array dimension and size are well controlled within the ranges as set forth below:

1) Ti film thickness: 5-5,000 nm

2) A width of the nano rod diameter, d: 0.5-500 nm

3) Nano rod inter-particle distance, D: 0.5-1000 nm

4) Nano rod height, H: 0.5-1000 nm

wherein d is a width, that is, a lateral dimension of the nano rod. For example, d can be the diameter of a substantially round nano rod. In another example, d can be a width of substantially rectangular nano rod.

On the other hand, the nano-hole array dimension and size can be well controlled by processes mentioned above. Specifically:

1) Ti film thickness: 5-5,000 nm

2) A width of the nano hole diameter, d: 0.5-500 nm

3) Nano hole inter-hole distance, D: 0.5-1000 nm

4) Nano hole depth: 0.5-1000 nm

wherein d is a width, that is, a lateral dimension of the nano hole. For example, d can be the diameter of a substantially round nano hole. In another example, d can be a width of substantially rectangular nano hole.

The nano-structured sensing surface shown in FIGS. 6 and 7H can be used in an optical probe for sensing chemicals. Referring to FIG. 8, an optical probe 200 includes an airtight cell 210 covered by a housing cover 205. The airtight cell 210 encloses a mechanism 220 with further structure details shown in FIG. 9 below. The cell 210 includes an air outlet 225 connected to a vacuum pump (not shown) to generate a vacuum space inside the cell 210. The cell further includes an air inlet 230 that has a valve to intake sample molecules as sniff trace chemicals for adsorbing onto the nano-structured surface as shown in FIG. 9. The optical probe 200 further includes an optical window 240 and a lens 250 for projecting laser beam as an incident beam to strike on the nano-structured surface to generate a scattering signal to carry out a light scattering molecule detection operation. The airtight cell 210 thus provides an isolated space as a mini-environment for light scattering measurement.

FIG. 9 is a perspective view of a mechanism 220 for supporting and operating the nano-structure surface that is fabricated with noble metal nano rods 120-N as shown in FIG. 6. The purpose of the nano-structured surface roller is to provide a mechanism to expose a fresh nano-structured surface 100, (one pocket size) to the air molecules inside the probe cell for any trace chemical molecules to adsorb onto the surface to provide surface-enhanced Raman scattering when laser light strikes on the surface. A nano-structured surface 260 can sense trace chemicals. The nano-structured surface 260 can be fabricated with photolithographic method, e-beam lithographic method, chemical reaction, PSL layer deposition followed by metal deposition, or special VLSI technology as described above. For example, the nano-structured surface 260 is compatible with nano-structured surface 100 described above and the nano holes and nano rods disclosed in relation to FIGS. 13A to 18. The nano-scaled noble metal particles are fabricated on a flexible metal foil or polymer material 270. To prevent the sensing surface from unexpected adsorbing molecules from air, the sensing surface is covered with a polymer or thin metal film 276. Further, to allow only a small portion of the surface to be exposed to intended sample molecules, the surface is structured such that each small area is surrounded with a circle, squared, rectangular, or any other type of shapes with sealing ridges 275. When a new measurement is to be made after sample molecules are pumped into the probe cell, the driving roller 280 and the thin film peel roller 285 can advance a distance equal to the sealed pocket size to peel off the thin film 276 cover and expose the fresh nano-structured surface 260 for molecules to adsorb onto it. The rollers 245, 280 rotate in the direction as shown by the arrows on the rollers 245, 280. The laser beam 250 enters the cell through an optical window and strikes the nano-structured surface 260. The reflected spectral light 255 is reflected off the exiting path by a reflecting mirror 290. The scattered light is collected by the objective lens 295.

FIG. 10 shows the structural details for the sealing ridges 275 surrounding and securing the nano-structured surface 260 in a pocket. In exposing the nano-structured surface 260 to the incident light 250, the thin film 276 is peeled off to allow the nano-structured surface 260 to receive the incident laser beam and to allow interested trace chemicals adsorbed onto the fresh sensing surface to generate a Raman scattering light for trace chemical detection. The peeling-off configuration and sensing process enable the trace chemical detection operation to effectively minimize sample contamination and allow a new detection operation to conveniently carry out every trace chemical measurement by applying a fresh nano structured surface.

Referring back to FIGS. 9 and 10, the scattered light can be collected by a lens 295 before exiting the probe cell through the exit window (not shown). The scattered light can be analyzed by a Raman spectrometer to produce a Raman spectrum. A mirror 290 can direct a reflected laser beam 255 away from the exit window to prevent the reflected laser beam 255 to add noise in the Raman scattering signal.

To enhance the molecular adsorption of the metal surface, a DC voltage source is provided and connected to the sensing surface to provide a positive or negative voltage on the surface (not shown in the figures). Controlling the voltage can selectively enhance certain molecular adsorption; thus, provide a biased mechanism to enhance Raman scattering signals for certain molecules of interest. Furthermore, in order to enhance the molecular adsorption of nano-structured sensing surface, a thermoelectric cooler is applied to cool sensing surface down to the region from 0° C. to 20° C., which many trace chemicals of interest are condensed onto the sensing substrate in this temperature region, so that to further maximize trace chemical molecules to adsorb onto the sensing surface, and that further effectively enhance the Raman scattering signal.

To further enhance Raman scattering signal from a nano-structured sensing surface, a polarized laser beam is applied, which either close to parallel to the sensing surface and/or one of the principal axes of the nano array, or close to perpendicular to the sensing surface. The incident angle of the laser beam is arranged such that the laser polarization direction is closely aligned to the nano rods axis direction, i.e., perpendicular to the sensing surface normal direction, or parallel to the sensing surface. Since many organic chemical molecules are of benzene-ring contained structure, such chemical molecules are expected to orient with its large ring structure that can be conveniently polarized for laying flatly on nano-rod edge surface, nano-rod top surface, or bottom surface between neighboring nano-rods.

To reduce Raman scattering noise, the voltage applied to the metal surface can be modulated with a known frequency to provide a mechanism for differential measurement, as described in more detail below.

An exemplified embodiment of the nano-structured noble metal surface roller is illustrated in FIG. 11. In this embodiment, a rotary wheel 300 is constructed to provide pocket of nano-structured surface for SERS or SERRS. The wheel is motorized (not shown in the figure) and controlled by an electro-mechanical device. Each time sample air is pumped into the probe cell, the motorized wheel can rotate a step while the covering thin film 310 is lifted to expose the pocket surface 320. A number of pockets can be fabricated on the wheel to provide multiple measurements without changing any parts. Similar to the embodiment shown in FIG. 9, a DC voltage is connected to the metal surface to provide a positive or negative voltage for surface adsorption enhancement. The mechanism for exposing the nano-structured surface sealed in each individual pocket is similar to that of a roller driven one as described in previous paragraphs. The wheel 300 is enclosed inside the probe cell. The laser beam strikes the surface that is exposed to sample air and reflects off from the surface. As described above, the reflected spectral light is directed away from the light dispersing and collecting optics. Only the scattering light is dispersed and imaged to a CCD camera for spectral analysis.

The SERS or SERRS detector, as disclosed above has a compact size enclosed in an airtight probe cell with a nano-structured sensing surface, configured for individual exposure. The probe as disclosed can be conveniently deployed in the field. The nano-structured surface is configured and partitioned as pocketed and film protected surface for very cost effective and economical implementations. The nano-structured sensing surface is covered under the film and therefore is protected from contamination before a trace chemical detection is performed. The ridges are effectively implemented to seal and securely attach the protective film onto the nano-structured surface to assure the nano-structured surface is free from contaminations. A mechanism is disclosed to lift the covering film to expose a small portion of the surface to sample and detect the molecules. The rollers as disclosed support and operate the nano-structured surface to expose only a single pocket at a time to control an accurate and effective operation of the detection processes. Also, the detection operation is performed with a continuously advanced fresh, uncontaminated surface for new SERS or SERRS measurement. The roller configuration further enhanced the film replacement process for more efficient chemical detection operations. The DC voltage as now applied to the nano-structured surface further enhances the adsorptions and sensitivity of trace chemical detection. In some embodiments, the voltage applied to the conductive layer supports the nano-structured surface can be modulated to provide a differential signal to further reduce noises. To enhance the molecular adsorption of nano-structured sensing surface, a thermoelectric cooler is applied to cool sensing surface down to the region from 0° C. to 20° C., which many trace chemicals of interest are condensed onto the sensing substrate with higher probability in this temperature region. Furthermore, in order to enhance the molecular adsorption of nano-structured sensing surface, a polarized laser beam is applied, either parallel to the sensing surface and/or one of the principal axes of the nano array, or perpendicular to the sensing surface.

This application further discloses additional methods of carrying out a chromatography operation, e.g., gas chromatography (GC) or a high-performance liquid chromatography (HPLC) operation, before a trace chemical sensing is performed. A chromatography process is a process to separate a mixture by distribution of the components of the mixture between a mobile and a stationary phase over time. The mobile phase may be a liquid or gas phase and the stationary phase may be a component attached to a column packing material. This application thus discloses a combined GC-Raman (or GC-SERS) sensing system or a combined HPLC-Raman (or HPLC-SERS) sensing system by first carrying out a classification by phase process, such as GC or HPLC, followed by detecting the trace chemicals by Raman scattering sensing process described above.

The detection sensitivity of the Raman scattering sensors can also be enhanced by that the surface electron-photon coupling effect and surface interference effect can be combined with the dimension of the nano-structured surface. Specifically, the electron mean-free path (MFP) on a gold or silver surface is about ten to fifty nano-meters as disclosed by Penn, D. R. in 1976 Phys. Rev. B13, 5248 and the Universal Curve (Physics at Surface, Andrew Zangwill, Cambridge University Press, 1988). The silver metal surface can be configured to have a nano-array with the scale to match the scale of the silver electron MFP. The physical properties of the silver nano-structured surface array demonstrate sudden significant changes when interacted with an incident visible polarized laser. The sudden changes of the physical properties can be quantified to correlate to the interaction between the photons and the electrons and other sub-atomic particles caused by the surface electron-photon-phonon coupling effect, surface interference effect, surface resonance effect, quasi-diffraction effect at the surface, and so on.

A MFP of an electron on a silver nano-structured surface is based on the Universal Curve as a function of the kinetic energy of that electron as tabulated below. Assuming the excited laser energy is transferred as kinetic energy to an electron on the Ag surface, the table below lists the MFP of the electron on a silver nano-structured surface for different laser wavelengths:

a) laser wavelength=375 nm, MFP≈50 Å

b) laser wavelength=532 nm, MFP≈100 Å

c) laser wavelength=785 nm, MFP≈220 Å

d) laser wavelength=1064 nm, MFP≈410 Å

Accordingly, the electron MFP at the Ag metal surface is in the range of 5-50 nm under the condition that the excited laser wavelength is in the range of 375-1064 nm. From above discussion, it can predict that the optimized and maximized SERS signal enhancement occurs under the condition that when the electron MFP is functionally matched by optimized several nano-structure parameters. These parameters include i) the diameter of the silver nano rod array or nano hole array d, ii) The inter-rod or the inter-hole distance on the nano-structured surface D, iii) the height of the nano rod array, or the depth of the nano hole array, or iv) any two of the above three parameters. The “functionally match” as described above may include the condition that Ag surface nano feature size(s) mentioned above is (are) approximately equal to, smaller than, integer numbers of, or with a special mathematical function to the estimated electron MFP of Ag metal. The functional match correlation can also be defined as by a functional relationship as characterized by the interaction between the photons and the electrons and other sub-atomic particles caused by the surface electron-photon coupling effect, surface interference effect, surface resonance effect, quasi-diffraction effect at the surface, an other inter-particle interactions.

Similarly, the matching of MFP of Ag electrons to the nano surface features can be extended to i) The Electron Wavelength. Consider that the electron wavelength is in the range of about 2 Å-200 Å at the surface of Ag metal, if the metal surface nano feature size matches that range, then, non-conventional physical phenomena may occur under that laser beam excitation, such as surface enhanced Raman scattering, then resulted Raman scattering can be significantly enhanced. ii) The Phonon Wavelength. Consider that the phonon wavelength is in the range of 2 Å-1,000 Å at the surface of Ag solid, if the metal surface nano feature size matches that range under the laser excitation, Raman scattering can be significantly enhanced. It should be noted that the phonons are defined as the quanta of energy of the normal vibrational modes of a crystal lattice or chemical bonding, and Raman spectrum records crystal lattice or chemical bonding vibration. iii) The Phonon Mean-Free Path. Consider that the phonon mean free path is in the range of about 2 Å-20 μm at the surface of Ag solid, if the metal surface nano feature size matches above range, then resulted Raman scattering can be significantly enhanced. Notice that the phonons are defined as the quanta of energy of the normal vibrational modes of a crystal lattice or chemical bonding, and Raman spectrum records crystal lattice or chemical bonding vibration. Then Raman scattering can be significantly enhanced by the interaction among the photons, the electrons, the phonon, and other sub-atomic particles caused by the surface electron-photon-phonon coupling effect, surface electro-optical interference effect, surface resonance effect, quasi-diffraction effect at the surface, and other inter-particle interactions.

Based on the above descriptions, considering the interaction between the incident laser and the nano-structured surface, the scattering sensing intensity can be further enhanced by applying the incident laser modulation to adjust the incident laser to have a glance incident angle such that the laser polarization direction is close to the direction of the nano-rod axis, i.e., perpendicular to the sensing surface or parallel to a sensing surface. The sensing performance can also be enhanced by shifting the wavelength of the excited laser with about half of Raman band width and applying a spectra difference analysis technique to filter out a large portion of the background noises or/and unwanted fluorescence signal from sample, sensing environmental and sensing system, which both are with very broad band width. In addition to the above techniques, an alternate method is an electronic signal differential method to further enhance the performance of the scattering sensing process by shifting the charged-couple device (CCD) detection pixel position then applying a spectra difference method to reduce noises of detection.

In some embodiments, nano structures such as nano holes or nano rods can include multiple layers in their structures. Referring to FIG. 12A, a multi-layer structure 1200A can include a substrate 1205, an adhesion layer 1210, structure layers 1215, 1220, 1225, 1230, and an upper layer 1250. The substrate 1205 can include a silicon substrate having an (100) or an (111) crystal plane orientation. The adhesion layer 1210 can for example include a conductive material that can include Ti, Ni, or Co. The adhesion layer 1210 can be formed by physical vapor deposition (PVD) on the substrate 1205. The adhesion layer 1210 can be electrically conductive, thermally conductive, or both electrically and thermally conductive. An exemplified thickness for the adhesion layer 1210 is 5 nm-5,000 nm, such as 10 nm to 100 nm.

The adhesion layer 1210 can provide several functions. It can provide adhesion to the substrate 1205. It can provide an electrical bias or a temperature bias to the nano structures to be formed to enhance to light scattering signal. It can also act as a thermal heat sink. During the fabrication, the adhesion layer 1210 can act as a stop layer for chemical etching (as described below) or a diffusion barrier layer.

An optional thermal bias layer 1215 can next be formed on the adhesion layer 1210. The thermal bias layer 1215 can be formed for example by PVD. The thermal bias layer 1215 can be made of Cr, Pt, Ru, a Ni—Cr alloy, NiCrN, a Pt—Rh alloy, a Cu—Au—Co alloy, an Ir—Rh alloy, or a W—Re alloy. The thickness of the thermal bias layer 1215 can be in the range of 5 nm to 10 μm, such as 10 nm to 1 μm. The thermal bias layer 1215 can perform different functions depending on the applications. For example, when the nano holes or nano rods are used for sensing trace chemicals in a Raman scattering, the thermal bias layer can be cooled to act as a heat sink for the nano holes or nano rods. Lower temperature can enhance the adsorption of the trace chemicals to the surfaces of the nano holes or nano rods. The thermal bias layer 1215 can also be heated after each chemical sensing measurement to release the molecules adsorbed on the surfaces of the nano structures such that the nano surfaces can be reused for the next chemical sensing measurement.

In some embodiments, the adhesion layer 1210 and the thermal bias layer 1215 can be formed by a single substantially uniform layer, which for example can be implemented by a layer of Ti or Ni.

Next, structure layers 1220, 1225, and 1230 can be formed on the thermal bias layer 1215. The structure layers 1220, 1225, and 1230 can have different material compositions A, B, C (thus the structure layers can be abbreviated by “ABC”). Alternatively, the structure layers 1220 and 1230 can have a same material composition “A” and the structure layer 1225 can have a different material composition “B” (the structure layers can be abbreviated by “ABA”).

Similarly, referring to FIG. 12B, the structure layers 1220-1235 can include three or more layers having different sequences such as “ABAB”, “ABAC”, “ABCA”, “ABCB”, and “ABCD”, etc., wherein “A”, “B”, “C”, and “D” each represents a different material composition. Additional, referring to FIG. 12C, the structure layers 1220-1245 can have different material sequences such as “ABCDEF”, “ABABAB”, “ABCABC”, etc., wherein “A”, “B”, “C”, “D”, “E”, “F” each represents a different material composition. The multi-layer structure can have a layer sequence of (AB)_(n), (AB)_(n)A, (ABC)_(n), (ABC)_(n)A, (ABC)_(n)AB, etc., in which n is an integer.

The structure layer 1220, 1225, 1230 . . . 1245 can include metallic materials such as Ag, Au, Cu, Pt, Al, Fe, Co, Ni, Cr, Ru, Rh, and Pd; Ag doped with chlorine or chloride and Au doped with chlorine or chloride; oxides such as TiO₂, SiO₂, Al₂O₃, Si₃N₄, Ta₂O₅, Zn oxide, Sn oxide, Sb oxide, Fe oxide, Ag oxide, Au oxide; and polymeric materials such as Ethylene Chlorotrifluoroethylene (ECTFE), Poly(ethylene-co-butyl acrylate-co-carbon monoxide) (PEBA), Poly(allylamine hydrochloride) (PAH), Polystyrene sulfonate

(PSS), Polytetrafluoroethylene (PTFE), Polyvinyl alcohol (PVA), Polyvinyl chloride (PVC), Polyvinyldene fluoride (PVDF), and Polyvinylprorolidone (PVP). The structure layer 1220, 1225, 1230 . . . 1245 can also include semiconductor materials such as GaAs, ZnS, ZnO, CdS, Er³ in SiO₂, InGaN, InGaN/GaN, AlGaAs, InAgAs, GaAs/GaAlAs, GaN, 4H SiC, AlN, GaN, AlGaN/GaN, InP, InAlAs/InGaAs, Cs, Rb, diamond, InAs, AlSb/InAs, AlGaAs/InGaAs, InAlAs, InGaP, SiGe, Al, Si, C, etc.

The structure layer 1220, 1225, 1230 . . . 1245 can have a thickness in the range of 0.3 nm to 2000 nm. The formations of the structure layer 1220, 1225, 1230 . . . 1245 can be implemented by PVD, chemical vapor deposition (CVD), MOCVD, atomic layer deposition (ALD), molecular beam epitaxy (MBE), electroplating, electrolysis plating, spin coating, and spray, which can selected depending on the material composition and applications of the nano structures to be formed.

Next, referring to FIG. 12A, the upper layer 1250 is formed on the structure layer 1230 to complete the multi-layer structure 1200A. The upper layer 1250 can be formed by a material that is adapted to form an impression when printed by a mold. Material suitable for the upper layer 1250 includes polymethyl methacrylate (PMMA). The upper layer 1250 can have a thickness in the range of 20-800 nm, such as 50-350 nm. Similarly, the multi-layer structures 1200B and 1200C can also be formed by respectively forming the upper layer 1250 on the structure layer 1235 and the structure layer 1245.

A mold (template or stamp) 1300, referring to FIGS. 13A and 13B, is next disposed over the multi-layer structure 1200. The mold 1300 can include a plurality of protrusions 1310 (or holes) facing the upper layer 1250 (the cross section shown in FIG. 13A is along the line A-A in FIG. 13B). The protrusions 1310 (or holes) can determine the shapes and dimensions of the holes (1320) to be formed. The mold 1300 can be made by etching a silicon wafer or Ni, Ti, Co, or Cr coated glass to produce the protrusions 1310 (or holes) with the proper dimensions and spacing (center to center) in between. The mold 1300 is pressed against the upper layer 1250, as shown in FIG. 13C, to make an impression in the upper layer 1250, as shown in FIGS. 13C and 13D. The impression includes a plurality of recesses 1255 (or protrusions) in the upper layer 1250.

The upper layer 1250 is next chemically etched to form a mask 1251. The upper layer 1250 is etched in the recesses 1255 and the un-imprinted portions. The etching is controlled till the portions of the upper layer 1250 under the recesses 1255 are etched through to form holes 1320 in the mask layer 1251. The upper surface of the structure layer 1225 is exposed in the holes 1320 in the mask 1251.

The structure layers 1215-1225 are next etched by chemical etchant to form holes 1320 through the multi-layer structure 1200 (FIGS. 13F and 13G). The mask 1251 is subsequently removed to form a multi-layer nano-hole array 1350 having the multi-layer structure 1200 with the plurality of holes 1320 (FIG. 13H). The holes 1320 can have diameters in the range of 0.5 nm-1,000 nm such as 5 nm-200 nm. The center to center spacing between the adjacent holes 1320 is in the range of 0.5 nm-1,000 nm, such as 5 nm-200 nm. At least a portion of the plurality of holes 1320 can be distributed substantially in a periodic array in the structure layers 1215-1225. The holes 1320 include side surfaces 1325 on the structure layers 1215-1225. The surfaces 1325 can be coated by a conductive material. Similarly, the holes can be formed in the multi-layer structures 1200B and 1200C using the steps described above in relation to FIGS. 13A-13H.

The cross-sectional shapes of the holes 1320 are determined by the shapes of the protrusions 1310 in the mold 1300. Examples of the cross-sectional shapes of the holes 1320 can include circles, triangles, rectangles, etc.

In some embodiments, a plurality of columns 1430 can also be formed in the multi-layer structures 1200A, 1200B and 1200C using steps similar to the steps shown in FIGS. 13A-13H. As shown in FIGS. 14A to 14C, a mold 1400 can include a plurality of recesses 1420 or protrusions. The recesses 1420 or protrusions determine the positions and the lateral dimensions of the columns 1430 to be formed in the multi-layer structures 1200. The mold 1400 can be used to imprint on the upper layer 1250 to produce an impression that has protrusions in the upper layer 1250 at locations where the columns 1430 are to be formed. The multi-layer structure 1200 is then undergone the similar steps of etching, mask formation, and etching the area other than holes or the holes through the mask to form a nano-column or hole array including the plurality of columns 1430 or hole on the adhesion layer 1210, or even down to the substrate 1205 with a depth of 1-10,000 nm. A multi-layer nano column array 1450 is formed. The cross-sectional shapes the columns 1430 are determined by the shapes of the recesses 1420 in the mold 1400. For example, the cross-sectional shapes of the columns 1430 can include circles, triangles, squares, rectangles, rectangles with semi-circle at the both end, ellipse, etc. The diameter d or dimension in X-Y is in the range of 0.3-500 nm, such as 5-200 nm. The spacing (center to center) D between adjacent columns 1430 can be in the range of 0.5 nm-1,000 nm, such as 5-200 nm. The height of the columns can be in the range of 0.5-1,000 nm, such as 5-200 nm. At least a portion of the columns 1430 can be distributed in a substantially periodic pattern.

The disclosed multi-layer nano structure (e.g. the multi-layer nano-hole array 1350 and the multi-layer nano-column array 1450) may enhance the signal of the scattered light in several mechanisms. With the illumination of an incident laser beam, the electron standing waves can be formed in the multiple layers and the substrate, which can enhance surface plasmon resonance and thus Raman scattering. The nano holes or the nano rods can effectively act as nano cavities for the electron resonance. A multi-layer nano structure comprising metal material may also enhance charge transfer between chemical molecules adsorbed onto the surface and the structural materials of the nano rods or nano holes, which can enhance Raman signal. The localized electronic field density (E-field) may also be increased due to the electron resonance in the nano cavities. The strength of Raman scattering signal is known to be proportional to the fourth power of the E-field. Furthermore, the electron standing wave inside the nano cavity may also emit photons which can act as secondary excitation sources. The number of the secondary excitation sources is determined by the number of the nano rods and nano holes in the nano array under an external laser excitation. Lights emitted from the secondary excitation sources can coherently excite the chemical molecules adsorbed onto the sensing surface which may form Raman laser, thus can further enhance Raman signal.

In some embodiments, a plurality of holes or columns can be formed in a multi-layer structure by etching the multi-layer structure using a mask formed by anodization. As shown in FIG. 15A, a multi-layer structure 1500A includes a same structure as the multi-layer structure 1200 except for an upper layer 1550 is formed on the top (similar to the steps disclosed in relation to FIG. 7C). The upper layer 1550 can be made of a metallic material such as aluminum with a thickness in the range of 0.1-10.0 μm, such as 0.5 to 5 μm. The upper layer 1550 can be anodized using the steps described in relation to FIG. 7D to form pores 1555 in the upper layer 1550. The anodization can involve applying a voltage bias across the upper layer 1550, the adhesion layer 1210 (that is conductive) and the silicon wafer substrate. The aluminum material in the upper layer 1550 can be converted to Al₂O₃ in the anodization process. The annealing and the anodization processing parameters can be controlled such as the pores 1555 can self-assemble to form a hexagonal array with targeted physical dimensions in a hard mask layer 1551. The pores 1555 expose the upper surface of the structure layer 1225. The structure layers 1215-1225 are then chemically etched through the pores 1555 in the mask layer 1551 to form a nano-hole array 1550 having a plurality of holes 1520 through the structure layers 1215-1225. The holes 1520 include side surfaces 1525. The mask layer 1551 can optionally be removed after the formation of the holes 1520.

In some embodiments, a mask layer can be formed on a multi-layer structure using a combination of imprinting and anodization methods. For example, a mold having protrusions or recesses can be pressed against the upper layer 1550 in the multi-layer structure 1500A to form an imprinted pattern in the upper layer 1550. A subsequent anodization process can use the recesses in the imprinted pattern as nucleation sites to form the pores 1555 in the upper layer 1550, which produces the mask layer 1551. Alternatively, a subsequent etching process can use the recesses in the imprinted pattern as starting locations to etch the pores 1555 in the upper layer 1550, which produces the mask layer 1551.

In some embodiments, a different type of multi-layer nano structures can be constructed using a multi-layer structure 1600, as shown in FIG. 16. The multi-layer structure 1600 can include a substrate 1605, a first layer 1610, an optional bias layer 1615, a second layer 1620, a third layer 1625, and a mask layer 1650. The substrate 1605 can include a silicon substrate having an (100) or an (111) crystal plane orientation. The first layer 1610 can provide adhesion to the substrate 1605. The first layer 1610 can include a metallic material such as Ti, Ni, Co, Cr, etc., which can be formed by physical vapor deposition (PVD) on the substrate 1605 with a thickness range from 5-50 nm, such as 10 nm. During the fabrication, as described below, the first layer 1610 can act as a stop layer for chemical etching or a diffusion barrier layer.

Optionally, the bias layer 1615 is formed on the first layer 1610. The bias layer 1615 can be formed for example by PVD. The bias layer 1615 can include a conductive material, which can include Cr, Pt, Ru, a Ni—Cr alloy, NiCrN, a Pt—Rh alloy, a Cu—Au—Co alloy, an Ir—Rh alloy, or a W—Re alloy. The thickness of the bias layer 1615 can be in the range of 5 nm to 10 μm, such as 10 nm to 1 μm. The bias layer 1615 can perform different functions depending on the applications. For example, when the nano holes or nano rods are used for sensing trace chemicals in Raman scattering, the bias layer 1615 can be cooled to act as a heat sink for the nano holes or nano rods for the purpose of enhancing light scattering signals. Lower temperature can enhance the adsorption of the trace chemicals to the surfaces of the nano holes or nano rods. The bias layer 1615 can also be heated after each chemical sensing measurement to release the molecules adsorbed on the surfaces of the nano structures such that the nano surfaces can be reused for the next chemical sensing measurement.

In some embodiments, the first layer 1610 and the bias layer 1615 can be formed by a single layer comprising materials such as Ti, Ni, Co, Cr, etc.

Next, the second layer 1620 is formed on the bias layer 1615 using techniques such as physical vapor deposition (PVD). The second layer 1620 can provide magnetic field to the molecules adsorbed on the nano surface structures. The second layer 1620 can include, but not limited to, Ti, Ni, Co, Cr, Fe, alloys such as a Ni—Cr alloy, NiCrN, a Pt—Rh alloy, a Cu—Au—Co alloy, an Ir—Rh alloy, or a W—Re alloy, metal oxide, or other materials. The second layer 1620 can have a thickness range from 5-50 nm, such as 10 nm.

The third layer 1625 is then formed on the second layer 1620. The third layer 1625 can have a thickness in a range between 0.5 μm to 5 μm, such as 2 μm. Suitable materials include Al or Zn.

Next, the mask layer 1650 is formed on or from the third layer 1625. The material for the mask layer 1650 is selected depending on the methods with which the nano structures are formed. For example, suitable materials can include polymeric, metal, metal alloy, and oxide materials such as Ti, Ni, Co, Ag, Au, Pd, Cu, Pt, Sn, Al, Fe, Cr, Rh, Ru, SiO₂, Al₂O₃, ZnO, Ti oxide, Sn oxide, Fe oxide, metal oxide, polymethyl methacrylate (PMMA), and other polymer materials.

Referring to FIGS. 17A and 17B, a mold 1700 is next placed over the multi-layer structure 1600. The mold 1700 can include a plurality of protrusions 1710 facing the mask layer 1650 (the cross section shown in FIG. 17A is along the line A-A in FIG. 17B). The mold 1700 can be made by etching a silicon wafer or Ni, Ti, Co, or Cr coated glass to produce the protrusions 1710. The mold 1700 is pressed against the mask layer 1650, as shown in FIG. 17C, which produces a plurality of recesses 1655 in the mask layer 1650 as shown in FIGS. 17C and 17D. The protrusions 1710 determine the shapes and the dimensions of, and the distance between the recesses 1655 (and thus the holes 1720 in FIGS. 17E-17G). Examples of the cross-sectional shapes of the holes 1720 can include circles, triangles, rectangles, etc.

The mask layer 1650 is next chemically etched to form a mask 1651 as shown in FIG. 17E. The etching is controlled till the portions of the mask layer 1650 under the recesses 1655 are etched through to form holes 1720 in the mask layer 1651 to allow the third layer 1625 is exposed in the holes 1720 in the mask 1651.

It should be understood that the mask layer with holes can be formed by other techniques than the mold imprinting method described above. For example, the third layer 1625 can be formed by an aluminum layer. The top portion of the aluminum layer can be anodized to form a mask layer comprising aluminum oxide having openings, as described above in relation with FIGS. 7A to 7F. In this implementation, the mask layer and openings in the mask layer are simultaneously formed by anodization. Nano holes can then be formed and adjusted by etching through the openings in the mask layer. In some embodiments, the openings in the mask layer can also be formed by electron beam irradiation.

The multi-layer structure 1600 is next etched by chemical etchant such that are the holes 1720 are through the bias layer 1615, the second layer 1620, and the third layer 1625 (FIGS. 17F and 17G). The upper surface of the first layer 1610 is exposed at the bottom of the holes 1720. The holes 1720 can have diameters in the range of about 1 nm and about 1,000 nm such as about 5 and about 200 nm. The center-to-center distance between the adjacent holes 1720 is in the range of about 1 nm and about 1,000 nm, such as about 5 and about 200 nm. The depths of the holes 1720 are in the range of about 1 nm and about 2,000 nm. At least a portion of the plurality of holes 1720 can be distributed substantially in a periodic or regular array such as a hexagonal, triangle or square array. The holes 1720 is defined in part by the internal surfaces 1725 on the bias layer 1615, the second layer 1620, the third layer 1625, and the mask layer 1650.

Next, multiple structure layers 1810 is deposited on the surfaces 1725 as shown in FIG. 18 using techniques such as PVD, chemical vapor deposition (CVD), MOCVD, atomic layer deposition (ALD), molecular beam epitaxy (MBE), E-Beam coating, electroplating, electrolysis plating, spin coating, and spray. The multiple structure layers 1810 can be in a sequence abbreviated by AB, ABC, ABA, ABAB, ABABAB, ABCABCABC, wherein each of the “A”, “B” and “C” indicates a layer of a different material composition as described above. The structure layers can be distributed in a periodic sequence for their respective material compositions. The thickness of each of these layers can range from about 1 nm up to 2500 nm, such as about 10 nm.

Materials suitable for the multiple structure layers 1810 include Ti, Ni, Fe, Co, Ag, Au, Cu, Pt, Sn, Cr, polymeric materials, alloy materials, and oxide material such as TiO₂, SiO₂, Al₂O₃, Fe oxide, Si₃N₄, Ta₂O₅, Zn oxide, Sn oxide, Sb oxide, Ag oxide, and Au oxide.

Materials suitable for the multiple structure layers 1810 also include GaAs, ZnS, CdS, InGaN, InGaN/GaN, AlGaAs, InAgAs, GaAs/GaAlAs, GaN, 4H SiC, AlN, GaN, AlGaN/GaN, InP, InAlAs/InGaAs, Cs, Rb, InAs, AlSb/InAs, AlGaAs/InGaAs, InAlAs, InGaP, SiGe, C, Si, and SiC. As a result, a multilayer nano structure 1800 is formed, which can be used as the nano-structured surface 260 in the optical probe 200 (FIGS. 8 and 9). FIG. 18 illustrates a specific example of multiple layers 1800 having layer sequence ABAB.

The foregoing description should be considered as illustrative only of the principle of the invention. The described devices may be configured in a variety of shapes and sizes and the scope of the invention is not limited by the dimensions of the described embodiments. Numerous applications of the present invention will readily occur to those skilled in the art. Therefore, the invention is not intended to be limited to the specific examples disclosed or the exact construction, operation or the dimensions shown and described. Rather, all suitable modifications and equivalents fall within the scope of the invention. For example, the number, the thicknesses, and the sequence of the layers can vary without deviating from the spirit of the present invention. 

1. A micro structure for sensing a substance using light scattering, comprising: a substrate; a first layer on the substrate, wherein the first layer comprises a metallic material; a second layer over the first layer; a mask layer over the second layer, wherein a plurality of holes are formed through the mask layer and the second layer, wherein the plurality of holes are defined in part by internal surfaces on the second layer and the mask layer, wherein widths of the plurality of holes are in the range of about 1 nm and about 1,000 nm; and two or more structure layers formed on the mask layer and the internal surfaces in the plurality of holes, wherein the two or more structure layers comprise different material compositions.
 2. The micro structure of claim 1, wherein the two or more structure layers comprise at least one of a polymeric material, a metallic material, or an oxide material.
 3. The micro structure of claim 1, wherein the material compositions of the two or more structure layers form a repetitive pattern.
 4. A micro structure for sensing a substance using light scattering, comprising: a substrate; a first layer on the substrate; a second layer over the first layer; a mask layer over the second layer, wherein a plurality of holes are formed through the mask layer and the second layer, wherein the plurality of holes are defined in part by internal surfaces on the second metal and the mask layer, wherein widths of the plurality of holes are in the range of about 1 nm and about 1,000 nm; and one or more structure layers formed on the mask layer and the internal surfaces in the plurality of holes, wherein the structure layer comprises at least one of a polymeric material, a metallic material, or an oxide material.
 5. The micro structure of claim 4, wherein the one or more structure layers comprise a material selected from the group consisting of Ti, Ni, Co, Ag, Au, Pd, Cu, Pt, Sn, Al, Fe, Cr, Rh, Ru, SiO₂, Al₂O₃, ZnO, TiO₂, SiO₂, Si₃N₄, Ta₂O₅, Zn oxide, Fe oxide, Sn oxide, Sb oxide, Ag oxide, Au oxide, and polymethyl methacrylate.
 6. The micro structure of claim 4, wherein the one or more structure layers comprise a material selected from the group consisting of GaAs, ZnS, CdS, InGaN, InGaN/GaN, AlGaAs, InAgAs, GaAs/GaAlAs, GaN, 4H SiC, AlN, GaN, AlGaN/GaN, InP, InAlAs/InGaAs, Cs, Rb, InAs, AlSb/InAs, AlGaAs/InGaAs, InAlAs, InGaP, SiGe, C, Si, and SiC.
 7. The micro structure of claim 4, wherein the first layer comprises material selected from a group consisting of Ti, Ni, or Co.
 8. The micro structure of claim 4, wherein the second layer comprises Ti, Ni, Co, Cr, Al, or Zn.
 9. The micro structure of claim 4, further comprising a third layer between the second layer and the mask layer, wherein the plurality of holes are formed through the mask layer, the third layer, and the second layer, wherein the third layer comprises Ti, Ni, Co, Ag, Au, Pd, Cu, Pt, Sn, Al, Fe, Cr, Rh, Ru, SiO₂, Al₂O₃, ZnO, TiO₂, SiO₂, Si₃N₄, Ta₂O₅, Zn oxide, Fe oxide, Sn oxide, Sb oxide, Ag oxide, Au oxide, or polymethyl methacrylate.
 10. The micro structure of claim 4, wherein the plurality of holes are defined in part by an upper surface of the first layer.
 11. The micro structure of claim 4, further comprising a bias layer between the first layer and the second layer, wherein the plurality of holes are formed through the mask layer, the second layer, and the bias layer.
 12. The micro structure of claim 11, wherein the bias layer is configured to receive a bias voltage to enhance adsorption of molecules on the one or more surfaces in the plurality of holes for Raman scattering sensing of trace chemicals.
 13. The micro structure of claim 4, wherein one or more surfaces of the one or more structure layers are configured to adsorb molecules of a trace chemical for detection of the trace chemical.
 14. The micro structure of claim 13, wherein the molecules are adsorbed from a liquid, sol gel, a gas, an aerosol, or a mixture of liquid, sol gel, gas, and aerosol.
 15. The micro structure of claim 4, wherein at least some of the plurality of holes are distributed substantially in a periodic array in the one or more structure layers.
 16. The micro structure of claim 4, wherein the center-to-center spacing between the adjacent holes in the plurality of holes is in the range of about 1 nm and about 1000 nm.
 17. The micro structure of claim 4, wherein depths of the plurality of holes are in the range of 1 nm and 2000 nm.
 18. The micro structure of claim 4, wherein the mask layer comprises aluminum, Al oxide, or PMMA.
 19. A method for making a micro structure for sensing a substance using light scattering, comprising: forming a first layer on a substrate; forming a second layer over the first layer; forming a mask layer over the first layer; forming a plurality of holes are formed in the mask layer and the first layer, wherein the plurality of holes are defined in part by internal surfaces on the second layer and the mask layer, wherein widths of the plurality of holes are in the range of about 1 nm and about 1,000 nm; and forming one or more structure layers formed on the mask layer and the internal surfaces in the plurality of holes.
 20. The method of claim 19, further comprising: forming a third layer comprising aluminum on the second layer, wherein the step of forming the mask layer comprises anodizing at least a portion of the third layer to form the mask layer, wherein the mask layer comprises aluminum oxide, wherein the plurality of nano holes are formed in the second layer and the third layer by etching the second layer and the third layer through the plurality of openings in the mask layer.
 21. The method of claim 19, wherein the step of forming a plurality of holes comprises: pressing a mold comprising a plurality of protrusions on to the mask layer; forming a plurality of recesses in the mask layer; and etching the mask layer and the second layer through the plurality of recesses to form the plurality of holes in the mask layer and the second layer. 